Downhole tools are used for operations inside boreholes of oil and gas wells. Downhole tools operate in a very harsh environment and must be able to withstand inter alia corroding fluids, high temperatures and high pressure.
To avoid unnecessary and expensive disturbances in the production of oil and gas, the tools deployed downhole have to be reliable and easy to remove from the well in case of a breakdown. Tools are often deployed at great depths several kilometers down the well, and removing jammed tools are therefore a costly and time-consuming operation.
It is known to control hydraulic engines in a hydraulic system by means of control valves and/or sequence valves, which are coupled in between the engines and the respective pumps. In downhole equipment control is limited for the user due to the special situation many kilometers down the borehole. Furthermore, the control of such equipment has to be independent of surface control in case of breakdowns in communication between equipment and surface, so that tools engaging the borehole wall or production casing by hydraulic means may still be retracted from the well in case of breakdowns. Therefore, a need for highly reliant control systems exists which may be advantageously utilised in the design of fail-safe downhole control systems.